<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>simple</title>
</head>

<body>
    <div class="app">
        <div class="header">xx</div>
        <div class="content">
            <div class="aside">x</div>
            <div class="main">xxxx</div>
        </div>
    </div>
    <style>
        body {
            margin: 0;
            padding: 0;
        }

        .app {
            /* visible height */
            height: 100vh;
            /* border: solid 2px red; */
            /* step1 父标签开启flex布局 */
            display: flex;
            /* step2 确定主轴方向 */
            flex-direction: column;
            /* step3 父标签整体调控子标签位置 */
            /* justify-content: start; */
            /* step4 用处不大 跳过 */
            /* align-items: center; */
        }

        .header {
            /* border: solid 2px green; */
            height: 64px;
        }

        .content {
            /* border: solid 2px green; */
            /* step5 剩余量分配给声明了flex的所有标签 */
            flex: 1;

            display: flex;
            /* flex-direction: row;
                justify-content: start; */
        }

        .aside {
            border: solid 2px darkorange;
            width: 260px;
        }

        .main {
            border: solid 2px salmon;
            flex: 1;
        }
    </style>
</body>

</html>